feat(router): add ParamMap.keys to get a list of parameters

tools/public_api_guard/router/router.d.ts

@@ -59,6 +59,9 @@ export interface CanLoad {
     canLoad(route: Route): Observable<boolean> | Promise<boolean> | boolean;
 }
 
+/** @stable */
+export declare function convertToParamMap(params: Params): ParamMap;
+
 /** @stable */
 export declare type Data = {
     [name: string]: any;
@@ -156,6 +159,7 @@ export declare class NoPreloading implements PreloadingStrategy {
 
 /** @stable */
 export interface ParamMap {
+    readonly keys: string[];
     get(name: string): string | null;
     getAll(name: string): string[];
     has(name: string): boolean;